Aggregates, which are essential ingredients of concrete, may be derived from natural sources with minimal processing or from naturally occurring materials that are heat treated. Aggregates may also be synthetic. Aggregates from natural sources, such as quarries, pits in ground, and riverbeds, for example, are generally composed of rock fragments, gravel, stone, and sand, which may be crushed, washed, and sized for use, as needed. Aggregates from natural materials that may be used to form aggregates include clay, shale, and slate, which are pyroprocessed, causing expansion of the material. OPTIROC and LECA are examples of commercially available expanded clay aggregates, for example. Synthetic aggregates may comprise industrial byproducts, which may be waste materials. LYTAG, for example, is a commercially available sintered aggregate comprising pulverized fuel ash (“PFA”), also known as fly ash. PFA is produced from the combustion of coal in power plants, for example.
Aggregates may be lightweight or normal weight. Lightweight aggregates (“LWAs”) have a particle density of less than 2.0 g/cm3 or a dry loose bulk density of less than 1.1 g/cm3, as defined in ASTM specification C330. Normal weight aggregates from gravel, sand, and crushed stone, for example, have generally bulk specific gravities of from about 2.4 to about 2.9 (both oven-dry and saturated-surface-dry), and bulk densities of up to about 1.7 g/cm3. High quality LWAs have a strong but low density porous ceramic core of uniform structural strength and a dense, continuous, relatively impermeable surface layer to inhibit water absorption. They are physically stable, durable, and environmentally inert. LWAs should also be nearly spherical, to improve concrete properties and provide good adherence to concrete paste. Suitable sizes for incorporation in concrete are in a range of from about 4.75 mm to about 25 mm, or 2.36 mm to 9.5 mm for coarse aggregates, in accordance with ASTM Specification C330. Smaller, fine aggregates, which are a byproduct of LWA production, may also be used, to replace sand in concrete, for example. For use in concrete, LWAs should have a sufficient crushing strength and resistance to fragmentation so that the resulting concrete has a strength of greater than 10 MPa and a dry density in a range of about 1.5 g/cm3 to about 2.0 g/cm3. Concrete containing LWAs (“LWA concrete”) may also have a density as low as about 0.3 g/cm3.
While LWA concrete may be 20-30% lighter than conventional concrete, it may be just as strong. Even when it is not as strong as conventional concrete, the LWA concrete may have reduced structural dead loads enabling the use of longer spans, narrower cross-sections, and reduced reinforcement in structures. The lower weight of the LWA concrete facilitates handling and reduces transport, equipment, and manpower costs. LWA concrete may be particularly useful in construction slabs in high rise buildings and in concrete arch bridges, for example. LWA concrete may also have improved insulating properties, freeze-thaw performance, fire resistance, and sound reduction. LWAs can also be used in the construction of other structures, in highways, and as soil fillers, for example.
Quarrying is the largest source of aggregates by volume in most countries. Despite the many advantages of LWAs, aggregate extraction is complicated by environmental and legal issues, availability, and transportation and other costs, for example.
Waste disposal is another area presenting significant environmental and legal issues. Due to the exhaustion of available landfill sites, the difficulties in acquiring new sites, the adverse environmental effects, and the costs of landfilling, disposal of waste materials has been a significant problem for many years. For example, a significant amount of incinerator bottom ash (“IBA”), which is the principal ash stream generated from municipal solid waste (“MSW”) incineration, is generated and needs to be disposed of. IBA accounts for about 75% to about 80% of the total weight of MSW incinerator residues. IBA comprises a heterogeneous mixture of slag, glass, ceramics, ferrous and nonferrous metals, minerals, other non-combustibles, and unburnt organic matter. The considerable amounts of IBA produced present significant disposal problems. When landfilled, heavy metals may leach from the IBA into the ground and underground resources. IBA is currently used in its raw form (without heat treatment) in the construction of fills and embankments, pavement base and road sub-base courses, soil stabilization, landfill cover, in bricks, blocks, and paving stones, and as fillers in particular applications. Although considered a relatively inert waste, leaching of heavy metals in these applications is possible. Concrete containing IBA is weaker than concrete incorporating LYTAG, for example. The IBA may also chemically react with cement, leading to swelling and cracking.
As mentioned above, sintered LWAs comprising PFA are commercially available under the tradename LYTAG. Electricity-generating coal power plants for example, produce large amounts of PFA, which is typically collected as fine-grained particulate material from the furnace in flue gases. ASTM C 618 defines two major classes of PFA, Class F and Class C, on the basis of their chemical composition. Class F PFA, which comprises siliceous and sometimes aluminous material, is normally produced from burning anthracite or bituminous coal and has little or no cementitious value. Class C PFA, which is normally produced from the burning of subbituminous coal and lignite, usually contains significant amount of calcium hydroxide (CaOH) or lime (CaO). Class C PFA has some cementitious properties. The majority of PFA produced is currently disposed of in landfills at a great cost and risk of leakage of heavy metals that could contaminate underground aquifers.
U.S. Pat. No. 4,120,735 to Smith discloses a method of producing a brick or similarly fired construction unit, such as a ceramic-type tile or vitrified pipe, comprising mixing at least 50% by weight inorganic, non-ferrous residue from municipal incinerators (which generally refers to incinerator bottom ash) with coal fly ash and a binder, such as sodium silicate. The mixture is shaped and then fired in three steps. First, the mixture is heated at 180° C. for one hour, to ensure that moisture in the mixture is evaporated. Then the mixture is heated in increments of 65° C./minute to 550° C. and held overnight, to burn off carbon. Then the mixture is fired at temperatures of from about 1,700° F. (926° C.) to about 2,000° F. (1,093° C.), to form the brick. Smith emphasizes that the addition of the incinerator residue to the coal fly ash lowered the firing temperature as compared to a coal fly ash brick. Smith states that the incinerator residue, instead of the coal fly ash, melts to produce bonding on cooling. Considerable fusion is said to take place between 1,700° F. (926° C.) and 1,750° F. (954° C.). Smith also reports better brick properties as the proportion of incinerator residues increase. A preferred composition is 50% to 60% incinerator residue, 1% to 4% binder, and the remainder coal fly ash. Based on the low firing temperature, it is believed that the incinerator residue comprised predominantly glass and possibly incinerator fly ash. Due to the reported strengths, it is also believed that Smith produced a vitrified brick, with a large glassy, amorphous phase. The brick has high strength and low porosity, as the melted glass components of the incinerator residue filled most pores.
Avoidance of landfill disposal of wastes, such as IBA and PFA, by developing alternative, reuse applications, is needed. The economic burdens and the risks of waste disposal make it advantageous to develop alternative techniques for converting wastes into revenue-earning products that reduce the demand for less accessible, non-renewable materials.